U.S. patent number 10,739,257 [Application Number 16/149,295] was granted by the patent office on 2020-08-11 for method and system for the relative referencing of a target gas in an optical measuring system for laser spectroscopy.
This patent grant is currently assigned to Axetris AG. The grantee listed for this patent is Axetris AG. Invention is credited to Torsten Platz, Sven Schlesinger, Andreas Wittmann.
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United States Patent |
10,739,257 |
Wittmann , et al. |
August 11, 2020 |
Method and system for the relative referencing of a target gas in
an optical measuring system for laser spectroscopy
Abstract
A method for operating an optical measuring system including a
wavelength-tunable temperature-stabilized laser light source for
measuring the concentration of a target gas in a measured gas,
wherein an instantaneous base current I.sub.DC_ZG,act corresponding
to a wavelength .lamda..sub.ZG of a target gas absorption line is
set so that a wavelength distance .DELTA..lamda..sub.DC defined
during calibration between a target gas absorption line for a
target gas and a reference gas absorption line for a reference gas
is maintained. During operation, a temperature difference in the
laser light source, defined in advance during calibration, between
the operating points selected at the time of calibration of the
reference gas, with a base current I.sub.DC_RG,cal, and the target
gas, with a base current I.sub.DC_ZG,cal, is maintained by
determining the required instantaneous base current I.sub.DC_ZG,act
for the target gas, as a function of an instantaneous base current
I.sub.DC_RG,act for the reference gas.
Inventors: |
Wittmann; Andreas (Giswil,
CH), Schlesinger; Sven (Sachseln, CH),
Platz; Torsten (Weggis, CH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Axetris AG |
Kagiswil |
N/A |
CH |
|
|
Assignee: |
Axetris AG (Kagiswil,
CH)
|
Family
ID: |
69947386 |
Appl.
No.: |
16/149,295 |
Filed: |
October 2, 2018 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200103342 A1 |
Apr 2, 2020 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S
5/12 (20130101); G01N 21/274 (20130101); G01J
3/10 (20130101); H01S 5/06808 (20130101); G01N
21/39 (20130101); G01J 3/433 (20130101); H01S
5/02415 (20130101); H01S 5/0427 (20130101); G01J
2003/2866 (20130101); G01J 2003/4334 (20130101); G01N
2201/069 (20130101) |
Current International
Class: |
G01N
21/39 (20060101); G01J 3/10 (20060101); H01S
5/068 (20060101); H01S 5/12 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Amara; Mohamed K
Attorney, Agent or Firm: Renner, Otto, Boisselle &
Sklar, LLP
Claims
The invention claimed is:
1. A method for operating an optical measuring system for measuring
concentration of a target gas in a measured gas, comprising a
wavelength-tunable temperature-stabilized laser light source,
provided that an instantaneous base current I.sub.DC_ZG,act
corresponding to a wavelength .DELTA..sub.ZG of a target gas
absorption line being set so that, after a calibration, a
wavelength distance .DELTA..lamda..sub.DC between the wavelength
.lamda..sub.ZG of the target gas absorption line for a target gas
and a wavelength .lamda..sub.RG of a reference gas absorption line
for a reference gas component is maintained for an instantaneous
base current I.sub.DC_ZG,act of the target gas and of an
instantaneous base current I.sub.DC_RG,act of the reference gas,
wherein carrying out the calibration of the measuring system with a
reference gas and a target gas, and during calibration:
establishing base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal
assigned to the gas absorption lines for the reference gas and for
the target gas; determining associated electrical powers
P.sub.DC_RG,cal and P.sub.DC_ZG,cal of the laser light source from
the base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal and
associated internal resistances R.sub.I_RG,cal and R.sub.I_ZG,cal;
finding a power difference .DELTA.P.sub.DC,cal by means of the
electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal and storing
this or equivalent variables in the measuring system, and during
operation of the measuring system, maintaining a temperature
difference in the laser light source, defined in advance during the
calibration, between operating points selected at a time of
calibration of the reference gas, with the base current
I.sub.DC_RG,cal ascertained during the calibration and the target
gas, with the base current I.sub.DC_ZG,cal ascertained during the
calibration, by way of determining a necessary instantaneous base
current I.sub.DC_ZG,act for the target gas, as a function of the
instantaneous base current I.sub.DC_RG,act for the reference gas,
wherein ascertaining an instantaneous electrical power
P.sub.DC_RG,act of the laser source assigned to the gas absorption
line for the reference gas from the instantaneous base current
I.sub.DC_RG,act and an instantaneous internal resistance
R.sub.I_RG,act; determining an instantaneous electrical power
P.sub.DC_ZG,act of the laser source assigned to the gas absorption
line for the target gas as the sum from the instantaneous
electrical power P.sub.DC_RG,act and the power difference
.DELTA.P.sub.DC,cal; and calculating the assigned instantaneous
base current I.sub.DC_ZG,act from the instantaneous electrical
power P.sub.DC_ZG,act of the laser source, taking the instantaneous
internal resistance R.sub.I_RG,act into consideration.
2. The method according to claim 1, wherein during operation of the
measuring system, taking into consideration changes in a base laser
temperature T.sub.L,cal of the laser light source by adapting the
power difference .DELTA.P.sub.DC,cal.
3. The method according to claim 1, wherein determining an internal
resistance RI of the laser light source of the reference gas and
the target gas from a respective slope of a voltage/current
characteristic of the laser light source associated with the
instantaneous base current I.sub.DC_RG,act or I.sub.DC_ZG,act.
4. An optical measuring system for measuring concentration of a
target gas in a measured gas, comprising a wavelength-tunable
temperature-stabilized laser light source, provided that an
instantaneous base current I.sub.DC_ZG,act corresponding to a
wavelength .lamda..sub.ZG of a target gas absorption line is set so
that, after a calibration, a wavelength distance
.DELTA..lamda..sub.DC between the wavelength .lamda..sub.ZG of the
target gas absorption line for a target gas and a wavelength
.lamda..sub.RG of a reference gas absorption line for a reference
gas component is maintained for an instantaneous base current
I.sub.DC_ZG,act of the target gas and of an instantaneous base
current I.sub.DC_RG,act of the reference gas, wherein the measuring
system is designed to be calibrated with a reference gas and a
target gas, and during calibration: base currents I.sub.DC_RG,cal
and I.sub.DC_ZG,cal assigned to the gas absorption lines for the
reference gas and for the target gas are established; associated
electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal of the laser
light source from the base currents I.sub.DC_RG,cal and
I.sub.DC_ZG,cal and associated internal resistances R.sub.I_RG,cal
and R.sub.I_ZG,cal are determined; a power difference
.DELTA.P.sub.DC,cal by means of the electrical powers
P.sub.DC_RG,cal and P.sub.DC_ZG,cal is found and this or equivalent
variables are stored in the measuring system, and wherein the
measuring system is designed in such that during operation a
temperature difference in the laser light source, defined in
advance during the calibration, between operating points selected
at a time of calibration of the reference gas, with a base current
I.sub.DC_RG,cal ascertained during the calibration and the target
gas, with a base current I.sub.DC_ZG,cal ascertained during the
calibration, by way of determining a necessary instantaneous base
current I.sub.DC_ZG,act for the target gas, as a function of an
instantaneous base current I.sub.DC_RG,act for the reference gas is
maintained, an during operation: an instantaneous electrical power
P.sub.DC_RG,act of the laser source assigned to the gas absorption
line for the reference gas from the instantaneous base current
I.sub.DC_RG,act and an instantaneous internal resistance
R.sub.I_RG,act is ascertained; an instantaneous electrical power
P.sub.DC_ZG,act of the laser source assigned to the gas absorption
line for the target gas as a sum from the instantaneous electrical
power P.sub.DC_RG,act and the power difference .DELTA.P.sub.DC,cal
is determined; the assigned instantaneous base current
I.sub.DC_ZG,act from the instantaneous electrical power
P.sub.DC_ZG,act of the laser source is calculated, taking the
instantaneous internal resistance R.sub.I_RG,act into
consideration.
5. The optical measuring system according to claim 4, additionally
comprising: a modulation device for providing a base current
I.sub.DC and a modulation current I.sub.AC for the laser light
source; a measured gas; a light detector, and an evaluation unit
connected to the light detector and the modulation device; a
voltage detector configured to detect a voltage U.sub.L present at
the laser light source; a temperature detector configured to detect
a laser base temperature T.sub.L; a resistance detector configured
to ascertain an internal resistance R.sub.I of the laser light
source, and a processor configured to control base currents Ix for
a reference gas and the target gas, wherein the laser light source
is designed to em it a laser beam of a wavelength .lamda..sub.DC
having a wavelength modulation amplitude .DELTA..lamda..sub.AC, and
the modulation device is designed to periodically vary the
wavelength of the laser light of the laser light source by way of
the absorption line for the reference gas and the absorption line
for the target gas at an operating point and, at the same time, to
modulate the same with a frequency and a settable amplitude, the
modulation device is directly connected to the laser light source;
the light detector is designed to detect the laser beam originating
from the laser light source after this has passed through the
measured gas, and to generate a reception signal, which is
dependent on an intensity of the laser light after it has passed
through the measured gas and is supplied to the evaluation unit;
wherein the voltage detector for detecting the voltage U.sub.L
present at the laser light source, the temperature detector for
detecting the laser base temperature T.sub.L, the resistance
detector for ascertaining the internal resistance R.sub.I of the
laser light source, and the processor for controlling the base
currents I.sub.DC for the reference gas and the target gas are
configured to cooperate so as to detect the voltage U.sub.L present
at the laser light source and determine the internal resistance
R.sub.I and, as a function thereof, control the base currents
I.sub.DC for the reference gas and the target gas using the
modulation device so as to keep the wavelength distance between the
reference gas and the target gas constant.
Description
TECHNICAL FIELD OF INVENTION
The invention relates to a method for operating an optical
measuring system for measuring the concentration of a target gas
component (ZG) in a measured gas, comprising a wavelength-tunable
temperature-stabilized laser light source, wherein an instantaneous
base current I.sub.DC_ZG,act corresponding to a wavelength
.lamda..sub.ZG of a target gas absorption line is set so that a
wavelength distance .DELTA..lamda..sub.DC defined during the
calibration between a target gas absorption line for a target gas
component (ZG) and a reference gas absorption line for a reference
gas component (RG) is maintained. The invention furthermore relates
to a system for carrying out the method.
DISCUSSION OF RELATED ART
Optical measuring systems for measuring the concentration of a gas
component in a measured gas, based on laser absorption spectroscopy
(LAS), are known from the prior art in a wide variety of
embodiments, as are a multitude of different methods for operating
such an optical measuring system, such as wavelength modulation
spectroscopy (WMS) and direct absorption spectroscopy (DAS). In
LAS, exact setting of the laser wavelength to the wavelength of the
absorption line for the target gas plays an important role in
general. During calibration, the laser wavelength is usually
defined by way of the operating point of the Peltier temperature
(base laser temperature) and the base current I.sub.DC of the laser
light source. However, long-term changes in the laser and the
influence of the outside temperature result in a different
wavelength for the selected operating point, which deviates from
the absorption wavelength of the target gas, even if the base
current I.sub.DC remains unchanged. Base current usually denotes a
direct current (DC) having a maximum at the absorption line, with
larger and smaller current values traversed around this, when the
absorption line is being scanned. As a consequence, these changes
also cause the sensor calibration to deviate from the specification
limits, which then often necessitates a re-calibration of the
optical measuring system.
So as to ensure that the wavelength of the target gas remains
accurate (in particular in the absence of the target gas), usually
one or more absorption lines for a reference gas are used, and the
line locking principle is applied. The reference gas is either
essentially permanently present in the process gas or is
accommodated somewhere in the sensor itself, enclosed in a cuvette,
for example. The cuvette can be implemented, for example, in the
optical measuring path or in a special optical reference path (this
requires a beam splitter and a further photodiode).
Often, it is not possible or useful to use the target gas itself as
the reference gas, for example when this is not stable over a long
time, when the line width associated therewith is too thin, which
results from too short an absorption path and leads to a low
absorption signal, or when this is too dangerous to produce or
during operation.
As an alternative, a different gas (reference gas), which has one
or more absorption lines in the vicinity of a target wavelength,
can be used instead of the target gas. Reference is made to EP 2
307 876 B1 by way of example. This patent uses two CH.sub.4 gas
absorption lines to exactly reference the CO gas absorption line in
the center. Often, however, the tuning range of the laser light
source is not sufficiently wide to accurately reference a target
gas absorption line in this way.
EP 2 307 876 B1 discloses a method for detecting at least one
target gas by way of laser spectroscopy, using a laser light source
having an emission wavelength that is monochromatic and tunable by
varying the operating temperature or the operating current. So as
to calibrate the wavelength scale of the laser light source
relative to the varied operating temperature or the varied
operating current, in the wavelength range of a band of the at
least one target gas, a first laser light source tuning step is
carried out across a first tuning width, wherein at least two
absorption lines for a reference gas and at least one absorption
line for the at least one target gas are present. Thereafter, a
second laser light source tuning step is carried out across a
second tuning width in the wavelength range of the band of the at
least one target gas, wherein the second tuning width is narrower
than the first tuning width, and wherein at least one of the at
least one absorption line for the at least one target gas is
present. The target gas and the reference gas are different gases.
The first tuning step for calibrating the laser current or the
laser temperature is carried out once with the absolute wavelength
scale, and the second tuning step for detecting the at least one
target gas is carried out consecutively several times. The second
tuning width is compared to a calculated absorption spectrum,
wherein non-iterative curve fitting using a linear regression
algorithm is employed so as to calculate the concentration of the
at least one target gas in one step. This procedure requires a
reference gas that has at least two absorption lines in the
vicinity of the target gas absorption line, which are within the
tuning range of the laser. Moreover, this procedure is
time-consuming since it requires that two reference lines be
detected and evaluated.
For activating a wavelength-tunable laser diode in a spectrometer
using no reference gas, it is known from DE 10 2013 202 289 A1 to
predefine a power-time function, in accordance with which the laser
diode is tuned periodically over a wavelength range, by
ascertaining a current profile, which is used to activate the laser
diode, from the power-time function and the measured values of the
voltage present at the laser diode. The current profile that is
used to directly activate the laser diode is generated by a control
unit as a function of the control deviation between the power
consumption (actual variable) of the laser diode and the predefined
power-time function (target variable), wherein the voltage present
at the laser diode and the current through the laser diode are
continuously detected, for example measured, and the power
consumption of the laser diode is ascertained continuously by
multiplying the measured current and voltage values. No reference
gas is used therein, and thus a different wavelength range is
detected when the temperature of the Peltier element (Peltier
temperature) on which a laser chip is disposed and/or the outside
temperature change, even though the power-time function is
predefined. This method is too imprecise for referencing a target
gas. Moreover, the entire voltage across the laser is measured,
which is too imprecise for determining the wavelength.
If only one reference gas line, which is not the target gas line,
is available for the use of a reference gas, it is essential to
ensure that the wavelength distance .DELTA..lamda..sub.DC between
the target gas and the reference gas is kept constant during
operation for different laser currents, and thus different laser
temperatures. Moreover, the distance must also remain constant when
the laser ages. In principle, it would be possible to simply scan a
certain wave range. However, the detection limit is drastically
limited due to present noise, resulting from optical interference
phenomena, for example, and the presence of absorption lines for
other gases in the measured spectrum. It is therefore important to
adhere to the wavelength distance very precisely.
The wavelength distance between the target gas and the reference
gas is typically set by a constant current distance .DELTA.I. Due
to the non-linear DC tuning behavior of the laser, a shift (drift)
of the operating point, for example as a result of the influence of
the outside temperature on the temperature stabilization or aging
of the laser/electronics, results in a distance error for the
wavelength distance. This results in the target gas component
(absorption line for the target gas) being presumed to be in an
incorrect location. In some instances, this results in considerable
measurement errors.
Proceeding from this, it is the object of the claimed invention to
propose a different, more precise, effective and easy to implement
option for keeping the wavelength distance between the target and
reference gas lines constant, despite changing laser properties,
such as operating current, operating temperature or long-term
drift.
SUMMARY OF THE INVENTION
This object is achieved according to the invention by a method for
operating an optical measuring system for measuring the
concentration of a target gas component in a measured gas having
the features of described herein and by a system in the form of a
measuring system described herein. Further advantageous embodiments
can be derived from the respective dependent claims.
The core idea of the invention is, instead of using a fixed current
difference value, to set the current difference in such a way that
a previously defined relative temperature difference is maintained
between the reference and target gas (peak) positions, which is to
say between the respective absorption lines, for the calculation of
the presumed wavelength position of the target gas. This
temperature difference is calculated from the difference of the
temperatures that are generated by the respective introduced
electrical powers at an internal resistance of the laser light
source at the peak positions of the target and reference gases at
the time of calibration. This relative temperature difference is
proportional to the wavelength distance of the absorption lines for
the target and reference gases, and thus this procedure ensures
that the target wavelength can always be exactly determined.
For example, this method advantageously allows applications in
which reference gas and target gas absorption lines are
significantly distanced from each other to be easily implemented.
Monitoring for methane leaks shall be mentioned here by way of
example, where, in addition to CH.sub.4, C.sub.2H.sub.6 (ethane)
also has to be measured to confirm that natural gas is involved.
The reference gas used is methane (which, in this case, also
represents the second target gas), which is accommodated in the
optical path of the laser in the sensor for this purpose. Depending
on the selected absorption lines, the distance between the
reference gas (methane) and the target gas (ethane) is between 0.6
and 1.0 nm here. Since this distance is considerable, it is no
longer possible to use a fixed current value. Additionally, there
is an advantage in that the calibration is particularly simple, as
compared to the conventional calibration methods known from the
prior art.
The abbreviations RG and ZG used hereafter denote a reference gas,
or a reference gas component in a gas, and a target gas, or a
target gas component in a measured gas. The designations DC and AC
used are common and thus known to a person skilled in the art as
designations for DC voltage/direct current and AC
voltage/alternating current. These refer to electrical currents,
voltages and/or powers and indicate the respective type. Moreover,
the abbreviation L is used for the laser light source or, in
general, for the laser. The abbreviations RG, ZG, DC, AC and L are
used for clarification in the claims and in the overall
description, and in particular in the formulas below. The base
current I.sub.DC denotes, in summary, all the base currents
corresponding to the respective operating states (calibration,
instantaneous base current) and associated gases (RG, ZG).
According to the invention, in the method, during operation, a
relative temperature difference in the laser light source, defined
in advance during the calibration, between the operating points,
selected at the time of calibration, of the reference gas (RG),
with a base current I.sub.DC_RG,cal, and the target gas component
(ZG), with a base current I.sub.DC_ZG,cal, is maintained by
determining the required instantaneous base current I.sub.DC_ZG,act
for the target gas component, as a function of an instantaneous
base current I.sub.DC_RG,act for the reference gas.
The calibration of the measuring system is preferably carried out
with the reference gas and the target gas. During the calibration,
the base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal, assigned to
the gas absorption lines for the reference gas RG and of the target
gas component ZG are established, the associated electrical powers
P.sub.DC_RG,cal and P.sub.DC_ZG,cal of the laser light source are
determined, and a power difference .DELTA.P.sub.DC, cal is found
therefrom, which is stored.
According to the invention, moreover, during operation of the
measuring system, the electrical power P.sub.DC_RG,act of the laser
source assigned to the gas absorption line for the reference gas is
then ascertained, the electrical power P.sub.DC_ZG,act of the laser
source assigned to the gas absorption line for the target gas ZG is
determined as the sum of the electrical power P.sub.DC_RG,act and
the power difference .DELTA.P.sub.DC, cal, and the assigned base
current I.sub.DC_ZG,act is calculated from the electrical power
P.sub.DC_ZG,act of the laser source thus determined.
In other words, this means that, during the calibration, the DC
laser currents of the peak positions of the reference gas and the
target gas are established, the electrical DC power dropping across
the internal resistance of the laser is ascertained for the two
peak positions by detecting the internal resistance of the laser at
the respective current position or, alternatively, an equivalent
variable, for example using lock-in technology. At the time of
calibration, the power difference .DELTA.P.sub.DC, cal of the DC
powers, or at least an equivalent variable, is ascertained and
stored in the sensor. The power difference is preferably determined
according to formula F 10 listed below. As an alternative, it would
also be possible to store the variables indicated in formula F 10
in the sensor.
During operation of the measuring system, the electrical DC power
dropping across the internal resistance of the laser is determined
for the instantaneous peak position of the reference gas. The
internal resistance measurement can be implemented, in particular,
by lock-in technology. Since the operating point may shift due to
drift, the electrical power P.sub.DC_RG,act is recalculated on a
regular basis (and thus cannot be assumed to be a fixed value).
Moreover, the instantaneous electrical power at the peak position
for the target gas is determined by adding the instantaneous power
for the reference gas peak and the power difference. From the
instantaneous power of the peak for the target gas, the DC current
for the peak position thereof is preferably ascertained according
to formula F 11 listed below. Thereafter, the measuring scan for
the target gas is carried out at the ascertained peak position for
the target gas, and the gas concentration is ascertained therefrom.
The base current defines the position at which the absorption
characteristic curve has a maximum. Scanning with larger and
smaller current values is carried out around this maximum to
ascertain the concentration.
The proposed method is used for the initial calibration of the
measuring system by the manufacturer and can also be utilized by
the user for potentially necessary subsequent calibration.
In wavelength modulation spectroscopy, as is customary, a
wavelength-tunable temperature-stabilized laser light source is
used, which periodically varies a central base wavelength of the
laser light of the laser light source by changing the base current
via a relevant absorption line for the gas component at an
operating point and, at the same time, modulates the same with a
frequency (f) and a determinable amplitude by way of a modulation
device. Using a light detector, the intensity of the laser light
after it has passed through the measured gas is detected. An
evaluation unit is used, which comprises means for the
phase-sensitive demodulation of a measuring signal generated by the
light detector at the frequency (f) and/or one of the harmonics
thereof, wherein the laser light source is operated in a
current-modulated manner with a base current I.sub.DC and a
modulation current I.sub.AC and emits a laser beam of the
wavelength having a wavelength modulation amplitude
.DELTA..lamda..sub.AC, and the wavelength modulation amplitude
.DELTA..lamda..sub.AC of the laser light is kept constant by way of
variable setting of the current modulation amplitude
.DELTA.I.sub.AC.
However, long-term changes in the laser may make it necessary to
adapt the base laser temperature. However, this also necessitates a
correction of the previously calculated power difference
.DELTA.P.sub.DC,cal. If the laser temperature is being varied,
formula F 15 should be preferred over formula F 11 for calculating
the base current I.sub.DC_ZG,act. For this purpose, the laser
temperature is stored at the time of calibration.
According to a preferred method step, the respective internal
resistances are required for calculating the electrical power at
the respective base currents (for the target and reference gases).
It should be noted that the exact internal resistance may iterate
only after multiple measurements.
The internal resistance R.sub.I of the laser light source is
preferably determined from a voltage/current characteristic curve
for the laser light source in which the voltage drop U.sub.L across
the laser light source is recorded as a function of the base
current I.sub.DC. The respective internal resistances are
ascertained at the peak positions of the reference gas and the
target gas. The voltage/current characteristic curve of the laser
light source is usually recorded for the first time during the
calibration of the optical measuring system. It is not necessary to
measure the entire curve, but only a region around the operating
point, so as to correctly determine the respective internal
resistance R.sub.I from the slope in this region. A multitude of
measurements can be carried out in the region, and the result can
be averaged. As an alternative, the slope can also be ascertained
using lock-in technology, which has the advantage that noise is
reduced. The resistance values are also ascertained during
operation on a regular basis, and preferably these are
re-determined during every concentration measurement.
During calibration and during regular operation of the optical
measuring system, an evaluation unit comprising lock-in technology
is preferably used for determining the concentration of a target
gas component, so as to achieve noise reduction in the known
manner, in particular in order to considerably lower the noise
caused by the 1/f signal. A lock-in amplifier, which is also
sometimes referred to as a phase-sensitive rectifier or carrier
frequency amplifier, is an amplifier for measuring a weak
electrical alternating signal, which is modulated with a reference
signal having a known frequency and phase. The device represents an
extremely narrow-band bandpass filter, thereby improving the
signal-to-noise noise ratio. The advantage when using such a device
is that DC voltages and AC voltages having different frequencies
and noise are efficiently filtered.
According to the invention, the method described above is based on
the formulas provided below. These are used during calibration
and/or during operation. Values from calibration are denoted by
".sub.cal" and values from operation are denoted by ".sub.act."
Heretofore, the instantaneous current position of the target gas
(ZG) is determined in relative terms by the position of the
reference gas (RG) using the following formula:
I.sub.DC_ZG,act=I.sub.DC_RG,act+.DELTA.I (F 1) where .DELTA.I is a
fixed current value, which is established during calibration:
.DELTA.I=I.sub.DC_ZG,cal-I.sub.DC_RG,cal (F 2) Due to the
aforementioned non-linear DC tuning behavior of the laser, this
procedure is not sufficiently exact for determining the position of
the ZG. It is apparent from
.lamda..times..times..times..lamda..times..times..times..times..times..ti-
mes. ##EQU00001## that a wavelength change can be described by a
temperature change, where n.sub.eff is the effective refractive
index, and L.sub.eff is the effective resonator length of the
laser. Taking into consideration that the first term on the right
side of the above formula is dominant over the second term, a
wavelength change is given by:
.DELTA..lamda..lamda..times..DELTA..times..times..times..times.
##EQU00002## So as to calculate the presumed position of the target
gas, instead of using a fixed current difference value (that was
determined during calibration according to formula F 2), the
current difference is set during ongoing operation in such a way
that a defined temperature difference .DELTA.T.sub.act is
maintained between the reference and target gas (peak) positions.
This results in the following temperature at the instantaneous
position of the target gas:
T.sub.ZG,act=T.sub.RG,act+.DELTA.T.sub.act (F 5) wherein the
temperature at the position of the reference gas peak is given by:
T.sub.RG,act=P.sub.DC_RG,actR.sub.th,act=I.sub.DC_RG,act.sup.2R.sub.I
RG,actR.sub.th,act (F 6) wherein the electrical power is converted
via the thermal resistance R.sub.th,act into the temperature
T.sub.RG,act, and the power is produced by the laser current
I.sub.DC_RG,act in the internal resistance R.sub.I RG,act.
The instantaneous temperature T.sub.ZG,act at the position of the
target gas peak is coupled to the sought laser current
I.sub.DC_ZG,act via the thermal resistance and electrical
resistance:
T.sub.ZG,act=P.sub.DC_ZG,actR.sub.th,act=I.sub.DC_ZG,act.sup.2R.sub.I
ZG,actR.sub.th,act (F 7)
If the laser temperature does not change during operation,
then:
.DELTA..times..times..DELTA..times..times..DELTA..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times. ##EQU00003##
The aforementioned temperature difference .DELTA.T.sub.cal is
calculated as the difference of the temperatures that are generated
by the respective introduced electrical powers at the internal
resistance at the peak positions of the target and reference gases
at the time of calibration. Since the temperature difference is
proportional to the wavelength difference between ZG and RG, this
procedure ensures that the target wavelength is always exactly
determined.
By inserting formulas (F 6), (F 7) and (F 8) in formula (F 5), the
sought laser current at the position of the target gas is
found:
.times..times..times..times..times..times..times..times..times..times..DE-
LTA..times..times..times..times..times..times..times. ##EQU00004##
wherein a power difference ("fixed power") results using the values
at the time of calibration:
.DELTA.P.sub.DC,cal=I.sub.DC_ZG,cal.sup.2R.sub.I
ZG,cal-I.sub.DC_RG,cal.sup.2R.sub.I RG,cal (F 10)
Assuming that the thermal resistance does not change during
operation (R.sub.th,act=R.sub.th,cal), the following results for
the peak position of the target gas:
.times..times..times..times..times..times..DELTA..times..times..times..ti-
mes. ##EQU00005##
It should be noted that the exact internal resistance R.sub.I
ZG,act is not known initially and would have to be measured (at the
location I.sub.DC_ZG,act). After running through the formula
several times, or after multiple measurements, however, the value
iterates very quickly, and the internal resistance is correctly
ascertained at the location I.sub.DC_ZG,act.
If, additionally, the operating point of the base laser temperature
T.sub.L changes (for example, if the Peltier temperature is adapted
during operation so as to compensate for excessively large
wavelength shifts on the current scale due to the adaptation of the
base laser temperature), from the constant relative temperature
change condition:
.DELTA..lamda..lamda..apprxeq..DELTA..times..times..times..times.
##EQU00006## then the following relationship for the instantaneous
temperature difference follows:
.DELTA..times..times..DELTA..times..times..times..DELTA..times..times..ti-
mes..times..times. ##EQU00007## wherein the temperatures are to be
used in Kelvin. The resulting current is given by:
.times..times..times..times..times..times..times..times..times..times..DE-
LTA..times..times..times..times..times..times..times.
##EQU00008##
If a change in the thermal resistance can be neglected, it again
follows that:
.times..times..times..times..times..times..DELTA..times..times..times..ti-
mes..times..times. ##EQU00009##
It is commonly known that each laser light source circuit diagram
can be replaced with an equivalent circuit, which comprises a laser
emitter (active zone) and an internal resistance R.sub.I connected
in series thereto. As soon as a base current I.sub.DC flows through
the laser light source, a voltage U.sub.L is present at the laser
light source, which in part drops across the laser emitter as a
partial voltage U.sub.E and across the internal resistance R.sub.I
as a partial voltage U.sub.RI, where U.sub.E typically has a value
of 0.9 to 1.1 V, depending on the laser type (having a wavelength
close to that common in the telecom industry). In the case of WMS,
additionally a modulation current I.sub.AC is added to the base
current; however, this does not affect this method since the
temperature changes average out over time.
The instantaneous DC power at the position of the reference gas
peak is calculated from the base current I.sub.DC_RG,act flowing
through the laser light source, and the voltage drop
U.sub.RI_RG,act occurring at the internal resistance R.sub.I_RG,act
as
P.sub.DC_RG=I.sub.DC_RG,actU.sub.RI_RG,act=I.sub.DC_RG,act.sup.2R.sub.I
RG,act (F 166)
The calibration of the optical measuring system takes place
substantially according to the common method that is routine to the
person skilled in the art by way of a known reference gas, which
may also be a target gas, and the actual target gas itself. So as
to establish the operating point of the laser light source, first
the temperature of the temperature-stabilized laser light source is
varied until the absorption signals for the reference and target
gases are detected at a desired operating point. Thereafter, the
power difference is ascertained and stored.
The system according to the invention is composed of a measuring
system for carrying out the method, comprising a modulation device
(4) for providing the base current I.sub.DC for the laser light
source, a receptacle for the measured gas, a light detector, and an
evaluation unit (6) connected to the light detector and the
modulation device, means for detecting the voltage present at the
laser light source, means for detecting the laser base temperature,
means for ascertaining the internal resistance of the laser light
source, and means for controlling the base currents I.sub.DC for
the reference gas (RG) and the target gas (ZG). It is particularly
preferred when the reference gas is disposed in the optical path of
the laser in the sensor, so that the laser beam emitted by the
laser light source first passes through the reference gas, and
thereafter through the measured gas, to the light detector. This
allows for extremely compact design of the entire measuring system
and production of easy-to-handle and easy-to-transport gas
detection devices.
The features and feature combinations mentioned above in the
description, and the features and feature combinations mentioned
hereafter in the description of the figures and/or shown only in
the figures, can be used not only in the respective indicated
combinations, but also in other combinations, or alone. It is not
necessary for all the features recited in the claims to be
implemented to carry out the invention. It is also possible to
replace individual features of the independent or dependent claims
with other disclosed features or feature combinations.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described again in more detail hereafter
based on the accompanying drawings. In the drawings, in schematic
illustrations:
FIG. 1 shows an optical measuring system suitable for carrying out
the method according to the invention;
FIG. 2 shows the equivalent circuit for the laser light source;
FIG. 3 shows a recorded voltage/current characteristic curve for
determining the internal resistance of the laser light source;
FIG. 4 shows the wavelength .lamda..sub.I as a function of the
current in a schematic illustration of the target gas position and
the reference gas position with a fixed current distance between
the reference gas peak and the target gas peak, at a certain
temperature, and the shift thereof due to temperature drift;
FIG. 5 shows a measuring curve for methane, serving as the
reference gas and a further target gas, and ethane, serving as the
target gas, at the target temperature (solid line) and after the
drift (dotted line) caused by a change in the laser
temperature;
FIG. 6 shows the distance error with respect to the target gas peak
compared to the drift of the reference gas peak, measured using a
fixed current difference and a fixed relative temperature
difference based on an example; and
FIG. 7 shows a flow chart for fixing the wavelength distance to a
reference wavelength.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically shows the basic design of an optical measuring
system 1 for measuring the concentration of a target gas component
ZG in a measured gas 2, based on wavelength modulation spectroscopy
(WMS). The measuring system 1 comprises a wavelength-tunable
temperature-stabilized laser light source 3, a modulation device 4,
a light detector 5, and an electronic evaluation unit 6. The laser
light source 3 emits a laser beam 7 of the wavelength
.lamda..sub.DC having a wavelength modulation amplitude
.DELTA..lamda..sub.AC. The modulation device 4 periodically varies
the wavelength of the laser light of the laser light source 3 by
way of a reference absorption line and a target gas absorption line
at an operating point and, at the same time, modulates the same in
a triangular manner with a frequency (f) and a settable amplitude.
This additionally comprises at least one DC and/or AC voltage
source or a DC and AC current source 4a, and associated modulation
means 4b for operating the laser light source 3. This can be used
to variably set the respective base current I.sub.DC and the
modulation current I.sub.AC. The modulation device 4 is connected
directly to the laser light source 3. The light detector 5 detects
the laser beam 7 originating from the laser light source 3 after
this has passed through the measured gas 2, and generates a
reception signal, which is dependent on the intensity of the laser
light after it has passed through the measured gas 2, and is
supplied to the evaluation unit 6. The evaluation unit 6 comprises
means for the phase-sensitive demodulation of a measuring signal
generated by the light detector 5 at the frequency (f) and/or one
of the harmonics thereof. The evaluation unit 6 comprises two
lock-in amplifiers 6a, 6b and a processing unit 6c. The processing
unit 6c evaluates the demodulated reception signal of the light
detector 5. Furthermore, an electrical connecting line 9 leads to
the lock-in amplifier 6b by way of which the voltage present at the
laser light source 3 is detected and the internal resistance is
determined. As a function thereof, the control unit controls the
base currents I.sub.DC and modulation currents for the reference
and target gases using the modulation device 4 so as to keep the
wavelength distance between the reference gas and target gas
constant. For this purpose, this comprises electrical control lines
8 and 10 to the modulation device 4. In the evaluation, primarily
the above-described formulas F 11 or F 15 together with F 10 are
used, among other things. Via the line 12, the Peltier temperature
is transmitted to the control unit 6. In this exemplary embodiment,
the reference gas RG is permanently present in the optical path
during calibration. The target gas component ZG is then introduced
into the measured gas chamber.
FIG. 2 shows the equivalent circuit for the laser light source 3.
The laser light source 3 can thus be arithmetically replaced with a
light emitter 3a, and an internal resistance R.sub.I, 3b connected
in series thereto. The laser light source 3 is operated in a
current-modulated manner with a base current I.sub.DC and a
modulation current I.sub.AC. Voltage U.sub.L is present at the
laser light source 3 and drops partially across the internal
resistance 3b as a partial voltage U.sub.RI and across the light
emitter 3a as a partial voltage U.sub.E.
FIG. 3 illustrates a current/voltage characteristic curve 13
recorded during the calibration of the optical measuring system 1
for determining the internal resistance R.sub.I RG of the laser
light source 3 at the location of the reference gas. The internal
resistance R.sub.I RG is determined from the relationship of the
current/voltage characteristics of the laser light source 3 at the
operating point 15 I.sub.DC_RG,cal for RG. For this purpose, the
current/voltage characteristic curve 13 (solid line) is provided
with a linear approximation line 14 (dotted line) at the operating
point 15 for determining the internal resistance R.sub.I RG at the
location of the reference gas. The slope of the approximation line
14 corresponds to the internal resistance R.sub.I, 3b at the
operating point 15 with R.sub.I RG. An analogous procedure is
applied at the location of the target gas for ascertaining R.sub.I
ZG.
In a schematic illustration, FIG. 4 shows, by way of example, the
wavelength position of a target gas and that of a reference gas at
a certain target temperature, wherein the two positions have a
defined wavelength distance .DELTA..lamda..sub.DC with respect to
one another, which via the DC tunability curve leads to a current
distance .DELTA.I. The absorption lines are shown with solid lines
for the target temperature and with dotted lines for a drift
induced by the reduced target temperature. The wavelength distance
.DELTA..lamda..sub.DC is typically set by a constant current
distance .DELTA.I. Due to the non-linear DC tuning behavior of the
laser, a shift (drift) of the operating point, for example as a
result of the influence of the outside temperature of the sensor on
the temperature stabilization or aging of the laser/electronics,
results in a distance error .DELTA.I.sub.F for the position of the
gas peak of the target gas GZ, resulting in measurement errors. The
figure schematically shows the positions of the reference gas and
the target gas at the time of calibration. Drift causes the
position of the reference gas line to be shifted. Due to the
non-linear relationship between the wavelength and the laser
current, a difference, which is to say an incorrect wavelength
having a wavelength distance error .DELTA..lamda..sub.F, that is, a
distance error between the calculated and the actual target gas
position, results for a fixed current distance .DELTA.I. In the
most favorable case, the error is small (typically <20 .mu.A),
and the sensor remains within the specification thereof in terms of
the accuracy of the concentration. If the error grows larger, the
error increases in a superlinear fashion. In the extreme case, the
peak will no longer even be in the tuning range (for example, the
target gas is no longer even measured in FIG. 4). The greater the
distance between the reference gas and the target gas, the lower
the allowed drift must be with this method; otherwise the distance
error is >20 .mu.A. The actual target gas position (associated
base current) is at pos. 1, and the target gas position calculated
via formulas F1 to F2 (associated base current) is at pos. 2. These
two target gas positions deviate from one another by the distance
error .DELTA.I.sub.F. It should be noted that the DC tunability
curve itself is shown unimpaired by drift. In reality, the DC
tunability curve itself can also change with long-term drift.
FIG. 5 shows, by way of example, a measuring curve, which is to say
a spectrum for methane CH.sub.4, having a lower peak, as the
reference gas, and ethane C.sub.2H.sub.6, having a higher peak, as
the target gas, at a certain target temperature (solid line G 1)
and a temperature deviating from the target temperature (dotted
line G 2). The deviation of line G 1 from line G 2 is caused by a
change in the laser temperature and is intended to illustrate the
influence of drift on the distance of the gas peaks. By way of
example, the distance error .DELTA.I.sub.F is shown which results
when, proceeding from a reference gas peak (here CH.sub.4) having a
fixed current distance, a target gas (ethane C.sub.2H.sub.6) is
measured at a slightly different laser temperature. A shift in the
reference gas peak by 0.96 mA results from the temperature
reduction. It is apparent that the actual distance between the
reference peak and the target gas peak has decreased from 1.669 mA
to 1.568 mA. As was already mentioned above, this is caused by the
non-linear DC tunability of the laser light source. A distance
error .DELTA.I.sub.F of approximately 100 .mu.A thus results. If
the proposed method is applied using a fixed relative temperature
change (formula (F 15)), the distance error .DELTA.I.sub.F is only
10 .mu.A (FIG. 6). As a result, the target gas signal can be
evaluated correctly.
FIG. 6 illustrates the dependence of the distance error
.DELTA.I.sub.F on the drift values based on an example. This shows
the distance error .DELTA.I.sub.F for the use of a fixed current
difference according to formula (F 1) and a fixed relative
temperature difference according to formula (F 15) for different
drift values. The values based on formula F 1 are shown as dots P
1, and the values based on formula F 15 are shown as crosses P 2.
It is apparent that, when using the proposed method, the distance
error .DELTA.I.sub.F of the base current I.sub.DC never exceeds 20
.mu.A, which is sufficiently small to ensure that the measured
concentration remains within the specifications.
FIG. 7 shows a flow chart for fixing the wavelength distance to a
reference wavelength. In a first method step S1, which is to say
during the calibration of the measuring system 1, firstly the DC
base currents of the peak positions of the reference gas and the
target gas are established and, secondly, the electrical DC power
dropping across the internal resistance of the laser is ascertained
for the two peak positions by determining the internal resistance
of the laser at the respective current position. Thirdly, the power
difference .DELTA.P.sub.DC,cal of the DC powers is ascertained and
stored in the sensor.
In the subsequent second method step S2, which is to say during
operation of the measuring system 1, initially the electrical DC
power dropping across the internal resistance of the laser is
ascertained for the instantaneous peak position of the reference
gas by measuring the internal resistance using lock-in technology,
then the instantaneous electrical DC power at the peak position for
the target gas is calculated by adding the instantaneous DC power
for the reference gas peak and the DC power difference between the
reference gas and the target gas ascertained during calibration.
Thereafter, the DC current for the peak position thereof is
ascertained, wherein according to formula F 11 or F 15 the internal
resistance of the target gas from the preceding measurement is used
for this purpose. If F 15 is used, additionally the ratio of the
instantaneous laser temperature to the calibration laser
temperature is used. The laser temperature is stored at the time of
calibration.
In the subsequent third method step S3, the actual measuring scan
for the target gas is carried out based on the ascertained peak
position for the target gas, and the gas concentration is
ascertained therefrom.
The method steps S2 and S3 are preferably carried out multiple
times in a loop, wherein the base current I.sub.DC_ZG for the
target gas position can be adapted between the runs, if the
instantaneous base current I.sub.DC_ZG, act deviates from the ideal
base current I.sub.DC_ZG, cal, which was ascertained during the
calibration of the optical measuring system.
* * * * *